The present invention relates to an organic thin film transistor array and a method of manufacturing the same, and more particularly to an organic complementary thin film transistor (organic CTFT) and a method of manufacturing the same, and more particularly, a desired predetermined circuit configuration using an organic thin film transistor.
In a thin film display device using liquid crystal or an organic electroluminescence (EL) device, a device for driving a pixel, a thin film transistor (TFT) that uses amorphous silicon or polycrystalline silicon for a channel, is used. In a current state, the TFT using amorphous silicon or polycrystalline silicon is unlikely to have plasticity. In addition, since a vacuum apparatus is used in a manufacturing process, manufacturing cost is generally increased. Accordingly, in these display devices, in order to realize a flexible display device or reduce the manufacturing cost, researches on the formation of TFTs used in a driving circuit using an organic material have been widely made. In the organic thin film transistor (organic TFT), a semiconductor layer configuring a channel can be formed by a simple process such as a printing method, a spray method or an inkjet method. Thus, the organic TFT can be manufactured with significantly low cost compared with the TFT using inorganic semiconductor. In addition, since a large-area light weight thin film display device or integrated circuit can be easily manufactured, the organic TFT is expected to be applied to liquid crystal displays, organic EL displays or an IC cards.
In order to realize the flexible display device, it is necessary to provide flexibility including a peripheral circuit which drives the pixels. The carrier mobility of about 10 cm2/V·s or more is required for the TFT used in the circuit which drives the pixel, and the organic TFT that only uses the low-molecular-weight organic molecules for the channel has proved to satisfy the above requirement. For example, in Science, Vol. 303, page 1644 (2004), the carrier mobility of 15 (cm2/V·s) is obtained in the organic TFT that uses single crystal of rubrene molecules for the channel. Also, in Applied Physics Letters, Vol. 84, page 3061 (2004), there has been reported the carrier mobility of 35 (cm2/V·s) at a room temperature for the single crystal of pentacene molecules which have been highly purified. However, in an organic TFT performance of which can be easily improved is composed of low-molecular-weight molecules, it is common to use vacuum deposition in the manufacturing thereof and the method is disadvantageous in view of the manufacture. Meanwhile, an organic TFT which can easily suppress manufacturing cost is composed of high-molecular-weight molecules, but it has a significantly low performance and thus is restrictively used.
As means for solving the above problem, there is a method of dissolving the low-molecular-weight molecules in a solvent and coating the solution to form a semiconductor layer of the channel. As to pentacene that is the most typical organic molecules as an applied example of the low-molecular-weight molecules to the TFT, for example, Journal of Applied Physics, Vol. 79, page 2136 (1996), and Journal of American Chemical Society, Vol. 124, page 8812 (2002) have reported a technique by which derivatives of pentacene molecules are synthesized, and a thin film is formed by using a solution in which the solubility with respect to the solvent is increased. Also, Synthetic Metals, Vol. 153, page 1 (2005) discloses a technique by which the pentacene molecules are directly solved in the solvent, and coated to form a thin film. In addition, Applied Physics letters, Vol. 84, page 3061 (2004) and Japanese Journal of Applied Physics, Vol. 43, page L315 (2004) disclose a procedure of solving pentacene molecules in the organic solvent.
In order to cheaply manufacture the organic TFT by coating, it is preferable that an electrode and an interconnection using a metal line as well as the organic semiconductor are prepared by coating. There is a method of coating fine particles of metal with an organic material so as to have the solubility with respect to the solvent, distributing metal ink or paste in which the fine particles are dissolved at predetermined places, performing a process at a predetermined temperature so as to eliminate the organic material, and forming the interconnection or electrode of metal. Currently, a method of forming an interconnection by printing silver or gold paste is established.
Meanwhile, in a field effect transistor (FET) using silicon, a complementary MOS (CMOS) device arranging two types of FETs in which a carrier for conducting a channel is an electron (n-type channel MOS) and a hole (p-type channel MOS) in series so as to reduce power consumption becomes an indispensable requirement of integration.
However, up to now, most of the organic TFTs are only operated as a p-type FET. The several causes thereof have been suggested, but are in controversy. For example, OYO BUTURI, Vol. 74, No. 9, 1196 (2005) discloses an example of an n-type channel and p-type channel organic TFTs, but the n-type and p-type TFTs are realized using separate organic semiconductor and an economically advantageous process is not disclosed. The principle configuring the n-type and p-type TFTs is not disclosed.
JP-A-2004-55654 discloses an organic semiconductor device in which source and drain electrodes are formed of materials having different work functions. For example, as the material of the source electrode used in the p-type organic semiconductor device, a material having a largest work function (metal such as gold, platinum, palladium, chrome, selenium, nickel, indium-tin-oxide (ITO), iridium zinc oxide (IZO), zinc oxide or an alloy thereof, tin oxide, copper iodide or the like) is preferably used. As the material of the drain electrode, metal or a compound having a work function smaller than that of the source electrode (metal such as silver, lead, tin, aluminum, calcium, indium, alkali metal such as lithium, alkaline earth metal such as magnesium or an alloy thereof, or an alkali metal compound, an alkaline earth metal compound or the like) is suitable. However, when the organic semiconductor material is in contact with the electrode material, charge exchange or charge screening occurs in the interface between the electrode and the organic semiconductor and thus the n-type/p-type is not determined only by the work functions of the electrodes.
JP-A-2004-211091 discloses organic semiconductor high-molecular-weight molecules for an organic thin film transistor, which represent both the p-type property and the n-type property by introducing a unit having a p-type semiconductor property (for example, a thiophene unit) and a unit having an n-type semiconductor property (for example, a thiazole ring) into a main chain, represent low off current using the same, and represent the both properties. However, although the property of the bulk can be defined, the electron structure of the semiconductor in the interface between the electrode and the organic semiconductor used in the FET and the interface between the insulator and the organic semiconductor cannot be determined. Thus, the property of the organic TFT is not defined.
JP-A-128028 discloses an organic FET using metal oxides having high conductivity by generating oxygen holes or interstitial metal in the lattices by deviating from stoichiometry ratios, (tin oxide, titanium oxide, germanium oxide, copper oxide, silver oxide, indium oxide, thallium oxide, barium titanate, strontium titanate, lanthanum chromate, tungsten oxide, europium oxide, aluminum oxide, or lead chromate), metal oxides having highest conductivity at their stoichiometry ratios (rhenium oxide, titanium oxide, lanthanum titanate, lanthanum nickel acid, lanthanum copper oxide, ruthenium copper oxide, strontium iridium acid, strontium chromate, lithium titanate, iridium oxide, or molybdenum oxide), conductive metal oxide (vanadium oxide, chrome oxide, iron calcium oxide, iron strontium oxide, strontium cobalt acid, strontium vanadium acid, strontium ruthenium acid, lanthanum cobalt acid, or nickel oxide), conductive metal oxide bronze (tungsten bronze (MxW03), MxM03, MxRe03 in which hydrogen atoms, alkali metal, alkaline earth metal or earth metal are included at a site where atoms are not present at the A position of the perovskite structure of tungsten oxide, molybdenum oxide, or rhenium oxide), as a semiconductor layer. In this case, the above metal oxide is only used as the semiconductor material and is not used as the electrode.
Meanwhile, in Physical Review Letters, 84, (26), page 6080 (2000), it is discussed that at the interface between the electrode and inorganic semiconductor, a method of deriving a vacuum level shift Δ from the physical constants of elements configuring the semiconductor and the electrode. If the vacuum level shift Δ is used, a Schottky barrier Φ for injection of carriers (electrons and holes) at the interface of the electrode and the inorganic semiconductor can be calculated and thus carrier injection velocity (the number of charges injected per 1 second) can be computed using an appropriate carrier injection mechanism such as the thermionic excitation model. That is, if the carrier is the electron, the Schottky barrier Φ is obtained by Equation 1.Φ=ΦM−χS+  Equation 1
Here, the vacuum level shift Δ is assumed to be a positive sign when the Schottky barrier Φ is increased in the case the electron is injected from the electrode to the semiconductor, φM denotes the work function of the electrode, and χS denotes the electron affinity of the semiconductor (a difference in energy between the vacuum level and the bottom end of a conduction band. In addition, according to Physical Review Letters, 84(26), page 6080 (2000), the Schottky barrier Φ is given by Equation 2.Φ=γB(φM−χS)+(1−γB)Eg/2  Equation 2
where, Equations 3 to 5 are as follows.γB=1−e2dMSNB/∈iι(Eg+κ)  Equation 3κ=4e2/(∈SdB)−2e2/(∈iιdMS)  Equation 4∈iι=1/(1/(2∈S)+1/(2∈M))  Equation 5
where, Eg denotes the band gap energy of the semiconductor, e denotes the elemental charge of the electron, dMS denotes a distance between atoms configuring the electrode and the semiconductor at the interface between the electrode and the semiconductor, NB denotes the number of bonds (bonding between atoms) per unit area at the interface between the electrode and the semiconductor, a denotes the number of closest atoms of configuration atoms in the interface direction of the interface between the electrode and the semiconductor, ∈S denotes a relative dielectric constant of the semiconductor, dB denotes a distance between the configuration atoms in the interface direction of the interface between the electrode and the semiconductor, and ∈M denotes a relative dielectric constant of the electrode. Since ∈M is infinity if the electrode is metal,∈iι˜2∈S  Equation 6
is used. If the electrode is not metal, Equation 5 may be used.
However, in the discussion of Physical Review Letters, 84, (26), page 6080 (2000), bonding between atoms at the interface is applied only to the interface between the electrode and the inorganic semiconductor of a chemical bond. In general, since the bond is relatively weak, it can not be applied to the interface between the electrode and the inorganic semiconductor.